Z-axis accelerometer

ABSTRACT

An accelerometer design is described. It operates by measuring a change in capacitance when one plate is fixed and one is mobile (free to accelerate). Unlike prior art designs where such changes are caused by variations in the plate separation distance, in the design of the present invention the plate separation distance is fixed, it being the effective plate area that changes with acceleration. A key feature is that the basic unit is a pair of capacitors. The fixed plates in each case are at the same relative height but the mobile plates are offset relative to the fixed plates, one mobile plate somewhat higher than its fixed plate with the other mobile plate being somewhat lower. Then, when the mobile plates move (in the same direction), one capacitor increases in value while the other decreases by the same amount. This differential design renders the device insensitive to sources of systematic error such as temperature changes. A process for manufacturing the design is described.

FIELD OF THE INVENTION

The invention relates to the general field of accelerometers withparticular reference to Z-axis units and methods for their manufacture.

BACKGROUND OF THE INVENTION

Accelerometers have wide applications such as for inertial navigationsystems, automotive safety, and missile control. Z-axis accelerometerscan be used to control side air bags, vehicle control and multi-axissensing systems. Normally, z-axis accelerometers are fabricated usingbulk micro-machined technology or stacked thin films. Such devices havelarge size and need double-side bonded three wafer processes. This canlead to stress control problems in the films which, in turn, causessticking problems during release.

The basic principle underlying the operation of units of this type isschematically illustrated in FIG. 1. Two plates, 14 a and 14 b areattached by springs 13 to support posts 12 and their capacitancerelative to the upper surface of substrate 11 is monitored. While thedevice accelerates in direction A, plates 14 a and 14 b are drawn closerto the upper surface of 11 and their mutual capacitance decreases inproportion to the rate of acceleration. Similarly, during accelerationin the −A direction, the capacitance increases.

The arrangement shown in FIG. 1 has the merit of providing a high levelof sensitivity. However it has the serious limitation that it cannotdistinguish capacitance changes due to acceleration from changes arisingfrom other causes such as temperature, and other possible systematicerrors. Since both capacitor plates respond to acceleration by moving inthe same direction, a differential design in which the capacitancechanges differently (preferably oppositely) for the two plates is notfeasible.

A routine search of the prior art was performed. The followingreferences of interest were found:

In U.S. Pat. No. 5,576,250, Diem et al. show how to fabricate off-setparallel plates for use in a sensor. The approach used is to form beamsby etching and then filling trenches, said filling material thenbecoming the beam. MacDonald et al. (U.S. Pat. No. 5,770,465) show atrench fill masking technique while Andersson (U.S. Pat. No. 5,723,790)teaches an accelerometer with multiple cantilever beams free to move inspecific directions, said motions being detected by means ofpiezoresistive sensors. Ishida et al. (U.S. Pat. No. 5,830,777) andDelapierre (U.S. Pat. No. 4,776,924) show other accelerometersprocesses.

SUMMARY OF THE INVENTION

It has been an object of the present invention to provide a Z-axisaccelerometer design.

Another object of the invention has been to provide a process formanufacturing said accelerometer.

A further object has been that said accelerometer be insensitive tomotion that arises from any cause other than acceleration (such astemperature changes).

These objects have been achieved by measuring a change in capacitancewhen one plate is fixed and one is mobile (free to accelerate). Unlikeprior art designs where such changes are caused by variations in theplate separation distance, in the design of the present invention theplate separation distance is fixed, it being the effective plate areathat changes with acceleration. A key feature is that the basic unit isa pair of capacitors. The fixed plates in each case are at the samerelative height but the mobile plates are offset relative to the fixedplates, one mobile plate somewhat higher than its fixed plate with theother mobile plate being somewhat lower. Then, when the mobile platesmove (in the same direction), one capacitor increases in value while theother decreases by the same amount. This differential design renders thedevice insensitive to sources of systematic error such as temperaturechanges. A process for manufacturing the design is described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a Z-axis accelerometer of the prior art

FIG. 2 is a plan view of an embodiment of the structure of the presentinvention.

FIG. 3 is a cross-section through part of FIG. 2, illustrating a keyfeature of the invention.

FIG. 4 is an isometric view of the general region that includes FIG. 2.

FIGS. 5-10 illustrate successive steps that make up the process of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The key feature of the present invention is that it provides adifferential design wherein two closely located detectors respond toacceleration by changing their capacitances in opposite directions. Ifthe capacitance of one decreases during acceleration then thecapacitance of the other will increase, and vice versa.

A plan view of a specific embodiment is shown in FIG. 2. This will beused to explain the operating principles that the invention teaches, butit will be understood that other Z-axis accelerometers, different inappearance from the unit of FIG. 2, could be built through applicationof these same principles.

The mobile portion (inertial body) of the unit which has a total massthat is less than about 10⁻⁹-10⁻⁷ Kg, is built around rectangular prism29 that includes projections 31 that are attached to U-shaped springs23. The latter are arranged so that two of them lie along the Xdirection and two lie along the Y direction. This ensures that movementof the inertial body in the X-Y plane is greatly restricted whileleaving the body free to move along the Z-axis with little restraint.The particular spring design shown in FIG. 2 is our preferred design,but other designs that accomplish the same purpose are readilyenvisaged.

FIG. 2 shows, in plan view, four capacitors (each one enclosed by brokenlines), the lower pair being marked as C1 and C2. Typically eachcapacitor has a capacitance of between about 0.1 and 10 pF. Thecapacitors have the form of comb structures which, in this example,comprise a mobile inside tooth 25 flanked by two fixed outside teeth 24.It will be understood that a wide range of similar structures could besubstituted for the one shown here without departing from the spirit ofthe invention. The fixed plates are attached to the substrate at theareas marked as 121 while the springs are attached at 21, the entirestructure shown in FIG. 2 being located within a cavity in the substrate(not explicitly shown but conceptually similar to cavity 15 in FIG. 1).This design results in a unit having a total thickness in the Zdirection that is less than about 5 microns. Each capacitor plate has anarea between about 300 and 3,000 sq. microns while the fixed and mobileplates are separated from each other by less than about 1 micron. Thisresults in a device that is capable of measuring accelerations betweenabout 1 g and 100 g to an accuracy between about 1 millig and 1 g (whereg is the acceleration due to gravity).

Also shown in FIG. 2 are the top edges of the four mobile capacitorplates 25 a, 25 b, 25 c, and 25 d. These are attached to rectangularprism 29 along its long sides, two per side. This can be more clearlyseen in FIG. 3 which is an isometric view of the part of 29 thatincludes mobile plates 25 a and 25 b. These can be seen to be attachedto side surface 49 of 29. It is a key feature of the invention that 25 aand 25 b are narrower than the full thickness of side 49, by an offsetamount 35. Thus 25 a is flush with the upper surface of 29 but ends adistance 35 from the lower surface while 25 b is flush with the lowersurface of 29 but ends a distance 35 from the upper surface.

The reason for offsetting the two mobile plates can be understood byviewing FIG. 4 which is a schematic cross-section made at 4—4 in FIG. 2.The plates in FIG. 4 are being viewed head-on, i.e. in the Y direction.The fixed plates 24 are all at the same level (on the Z-axis) but themobile plates 25 are seen to be offset by an amount 35, in oppositedirections relative to the fixed plates. Thus, when the structure isgiven an acceleration A in Z-direction, both mobile plates will movedownwards. Because of the two opposite offsets, the capacitance ofcapacitor C1 (a plates) will increase while the capacitance of capacitorC2 (b plates) will decrease by the same amount.

Since the capacitance changes on acceleration will always be equal andopposite, any situation in which the capacitances of both capacitors arefound to simultaneously increase (or decrease) will be known to be dueto systematic error such as dimensional changes due to temperaturevariations. Because of this feature, accelerometers designed accordingto the teachings of the present inventions have a low temperaturedependence.

We now present a process for manufacturing the accelerometer describedabove. Referring now to FIG. 5, the manufacturing process begins withthe provision of substrate 51 which is preferably of single crystalsilicon, but other materials such as polysilicon could also have beenused. On one side of 51 (arbitrarily chosen here to be the right side),pedestal 52 is formed by masking the surface with etch mask 53 and thenetching. Depending on etch conditions, pedestal 52 may have vertical orsloping sidewalls. On the left side of 51 trench 62 is formed by maskingthe surface with etch mask 63 (as seen in FIG. 6) and then etching.Depending on etch conditions, trench 62 may have vertical or slopingsidewalls.

Next, as shown in FIG. 7, the surface is coated with protective layer 73which is then patterned according to the shapes seen in FIG. 2 so as todefine the mobile and immobile parts of the device, including springs31. The patterning procedure ensures that left comb tooth 25 a ofcapacitor C1 is located over trench 62 while teeth 24 a are located tolie outside trench 62. Similarly, right comb tooth 25 b of capacitor C2is located over pedestal 52 while teeth 24 b are located to lie outsidepedestal 52

Referring now to FIG. 8, protective layer 73 is used as a mask while thesubstrate is etched. Our preferred material for layer 73 has beensilicon oxide, but other materials (acting as hard masks) such assilicon nitride or selected metals could also have been used. Thisresults in the formation of trench 162, which has vertical sidewalls,within which the original trench 62 now lies. In addition, pedestals 24a, which lie outside trench 62 and which will be connected to theimmobile part of the structure, and pedestal 25 a, which lies inside 62and will be connected to the mobile part, are formed. Additionally, as aresult of etching with layer 73 as mask, trench 152, having verticalsidewalls, is formed. The original pedestal 52 lies within trench 152.Pedestals 24 b, which lie outside pedestal 52, and which will beconnected to the immobile part of the structure, and pedestal 24 a,which arises from the top of 52, and will be connected to the mobilepart, are also formed.

Referring now to FIG. 9, with mask layer 73 still in place, all verticalsurfaces are selectively coated with second protective layer 91. Ourpreferred material for layer 91 has been silicon oxide but othermaterials such as silicon nitride or selected metals could also havebeen used. Then, as illustrated in FIG. 10, the substrate isisotropically etched until the mobile parts are released from directcontact with the substrate, remaining connected thereto only throughsprings 23 (see FIG. 2). Our preferred etchant for performing theisotropic etching has been a mix of sulfur hexafluoride (SF₆) and xenondifluoride (XeF₂). As result of this release etching step, the left andright comb structures are transformed into left and right capacitorssuch as C1 and C2 in FIG. 2. After the isotropic etching step, allprotective layers (73 and 91) are removed. Performance problems couldresult if these layers are left in place.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

What is claimed is:
 1. A process for manufacturing a Z-axisaccelerometer, having a mobile part and an immobile part, comprising:providing a substrate having left and right areas; within the rightarea, forming a first right pedestal that extends upwards from a firstupper surface; within the left area forming a first left trench thatextends downwards from said first upper surface; coating with a firstprotective layer and then patterning the protective layer to define themobile and immobile parts, said parts being connected by a springstructure intended to inhibit movement of the mobile part in a planeparallel to said upper surface; further patterning said first protectivelayer to define left and right comb structures, the left comb structurehaving at least one tooth located over the first left trench andconnected to the mobile part, and at least one tooth located outside thefirst left trench and connected to the immobile part, the right combstructure having at least one tooth located over the first rightpedestal and connected to the mobile part, and at least one toothlocated outside the first right pedestal and connected to the immobilepart; using the protective layer as a mask, etching the substrate,thereby forming: a second left trench within which lie the first lefttrench and at least two pedestals, one pedestal extending upwards frominside the first left trench and connected to the mobile part, and onepedestal extending upwards from outside the first left trench andconnected to the immobile part, and a first right left trench withinwhich lie the first right pedestal and at least two other pedestals, oneof the other pedestals extending upwards from within the first rightpedestal and connected to the mobile part, and one of the otherpedestals extending upwards from outside the first right pedestal andconnected to the immobile part; with the protective layer in place,selectively coating all vertical surfaces with a second protectivelayer; isotropically etching the substrate until the mobile part isreleased from direct contact with the substrate, remaining connectedthereto only through said spring structure; and thereby transforming theleft and right comb structures into left and right capacitors eachhaving mobile and immobile plates, said mobile plates being limited bythe springs to motion normal to the upper surface, whereby motion byboth mobile plates, relative to the immobile plates, results in anincrease in capacitance by one of the capacitors and a decrease incapacitance by the other capacitor.
 2. The process recited in claim 1wherein the first right pedestal has sloping sidewalls and a height ofbetween about 1 and 5 microns.
 3. The process recited in claim 1 whereinthe first left trench has sloping sidewalls and a depth of between about1 and 5 microns.
 4. The process recited in claim 1 wherein the secondleft trench has vertical sidewalls and a depth of between about 3 and 30microns.
 5. The process recited in claim 1 wherein the first righttrench has vertical sidewalls and a depth of between about 3 and 30microns.
 6. The process recited in claim 1 wherein the substrate isselected from the group consisting of monosilicon, polysilicon, andmetals.
 7. The process recited in claim 1 wherein the first protectivelayer is selected from the group consisting of silicon oxide, siliconnitride, and metals.
 8. The process recited in claim 1 wherein thesecond protective layer is selected from the group consisting of siliconoxide, silicon nitride, and metals.
 9. The process recited in claim 1wherein the step of isotropic etching further comprises using a mix ofSF₆ and XeF₂.
 10. The process recited in claim 1 further comprising,after isotropic etching, removing all protective layers.